Modern experimental psychology started in the 1800s with the research of Wilhelm Wundt. Psychology in the United States is generally traced back to the work of William James. There have been many different theoretical approaches to psychology since then. Probably the most famous psychologist is Sigmund Freud, whose work spawned the psychodynamic theories of human behavior. Somewhat in reaction to Freud’s work are the behavioral approaches to behavior (with Ivan Pavlov, John Watson, B. F. Skinner, and Edward Thorndike as some of the major theorists behind behaviorism). The cognitive theories came next. All in all, there are many different theoretical approaches to the psychology of behavior and most psychologists today are influenced by each approach. For a slightly more in-depth outline of the history of psychology, please view the pdf slides below.
Note: I’ve included links to Wikipedia for all of the above researchers because I believe the information on Wikipedia is mostly accurate and fair. The site also allows the information to be easily modified if any of it is inaccurate.